1. Field of the Invention
This invention relates generally to rotary seals for establishing sealing between a structure and a relatively rotatable surface for lubricant retention and environment exclusion. More particularly the present invention relates to resilient rotary shaft seals having geometry for manipulating interfacial contact pressure within the dynamic sealing interface to interact with lubricant during rotation of a relatively rotatable surface for the purpose of enhancing lubricant film thickness across the sealing interface and for causing lubricant movement toward the environment for ensuring adequate lubrication of the entire dynamic sealing surface to extend the service life thereof, and having geometry that provides for resistance to becoming skewed or twisted within the seal groove, thereby minimizing skew-induced wear caused by impingement of abrasive contaminants present in the environment.
2. Description of the Prior Art
The prior-art hydrodynamically lubricated rotary seal designs of U.S. Pat. Nos. 4,610,319, 5,230,520, 5,678,829, 5,738,358, 5,873,576 and 6,036,192 are known in the industry by the registered trademark xe2x80x9cKalsi Sealsxe2x80x9d, and pertain to seals that are typically used to partition a lubricant from an environment and hydrodynamically lubricate the dynamic sealing interface in response to rotation of a mating shaft.
Commercial seals are presently being manufactured and sold under U.S. Pat. Nos. 4,610,319, 5,230,520, 5,678,829, 5,738,358, all of which employ a dynamic sealing lip defining a dynamic sealing surface. The dynamic sealing lip has angulated hydrodynamic waves on its lubricant side for hydrodynamically wedging lubricant into the dynamic sealing interface, and has an abrupt exclusion edge on its environment side for excluding the environment, per the teachings of U.S. Pat. No. 4,610,319. The waves cause the dynamic sealing lip to have a variable thickness, being thinnest at the low point of the waves, and being thickest at the high point of the waves.
The lubricant film in the dynamic sealing interface is thin, and not uniform in thickness across the width of the dynamic sealing interface. The resulting lubrication of the dynamic sealing surface is uneven, and surface asperity contact and resulting rubbing wear sometimes occurs between the dynamic sealing surface and the shaft. The greatest film thickness occurs toward the lubricant side of the dynamic sealing interface, and the least occurs toward the environment side.
The angulated hydrodynamic waves cause the lubricant-side edge of the dynamic sealing surface to have a series of waves; this edge is sometimes called the wavy hydrodynamic edge. Film thickness and resulting lubrication of the dynamic sealing surface is greatest in the region of the waves because a large portion of the lubricant wedged into the dynamic sealing interface at the leading edges of the waves simply leaks out at the trailing edges, and only a much smaller portion travels toward the abrupt exclusion edge. As a result, the environment-side of the dynamic sealing surface is less well lubricated than the lubricant side, and more prone to wear and overheating.
If the lubricant is at higher pressure than the environment, the seal begins to bulge or protrude in to the extrusion gap between the seal housing and the shaft due to bending, shear and other stresses in the dynamic sealing lip caused by the pressure; this phenomenon is called xe2x80x9cextrusionxe2x80x9d by the seal industry. These stresses are highest at the thinnest points of the dynamic sealing lip; i.e. the low point of the waves.
Extrusion can lead to damage known as xe2x80x9cnibblingxe2x80x9d or xe2x80x9cextrusion damagexe2x80x9d, which is caused by cyclic stressing of the material protruding into the extrusion gap, causing it to fatigue and break away from the sealing element. The cyclic stress is typically caused by dynamic fluctuations in the size of the extrusion gap due to lateral shaft motion, causing high variations in the compression of any protruding material.
Localized extrusion damage promotes abrasive ingestion, and can also partially interrupt the hydrodynamic film causing the seal to run hotter and suffer premature compression set and surface embrittlement. Extrusion damage, and wear damage caused by environmental abrasives, can progressively destroy the dynamic sealing surface, beginning at the abrupt exclusion edge and progressively working toward the low point of the waves. Once the damage reaches the low point of the waves, the seal no longer blocks the leakage path, and fails.
Because the dynamic sealing lip has variable thickness, the seals have less than perfect compressive stability and can become locally twisted and skewed, especially when the pressure of the lubricant and the environment is substantially balanced. U.S. Pat. Nos. 5,230,520, 5,873,576 and 6,036,192 are directed at improving compressed stability.
The seal is radially compressed when installed, which also causes circumferential compression, which is effectively increased by thermal expansion. Since the seal circumference is relatively long compared to the seal cross-section, circumferential compression can cause buckling in a manner similar to the classic textbook example of a long, slender structural column under compressive loading. (Lubricant pressure can be used to prevent buckling.) Twisting related to lip width variation can help to initiate such buckling, which can cause the abrupt exclusion edge to acquire a skewed position. This can promote abrasive wear, particularly if the lubricant end of the seal is twisted toward the shaft at the location of skew, causing interfacial contact pressure to increase near the wavy hydrodynamic edge and decrease near the abrupt exclusion edge, which diminishes hydrodynamic lubrication and environmental exclusion.
Since there is more material at the thickest point of the dynamic sealing lip, when one attempts to axially constrain prior art seals to help control skew-induced wear, part of the differential thermal expansion between the seal and the housing is relieved circumferentially, causing material displacement from the thickest point of the lip to the thinnest point of the lip, thereby diminishing hydrodynamic lubrication. Elevated environment pressure can worsen the problem, apparently by causing higher contact pressure near the wavy hydrodynamic edge.
Testing has shown that prior art seals perform better when the lubricant pressure is higher than the environment pressure. When the environment pressure is higher than the lubricant pressure, the prior art dynamic sealing lip deforms unevenly due to its varying width, causing the environment-side edge to become non-circular and more prone to abrasion by the environment.
U.S. Pat. No. 6,036,192, which has not yet been commercialized, discloses a skew and twist resistant rotary seal assembly comprising a structure having a circular seal groove having first and second spaced seal groove walls and a relatively rotatable surface, combined with a circular body of sealing material being located within said circular seal groove and having a part thereof projecting from said circular seal groove, with at least one circular dynamic sealing projection extending from said circular body of sealing material and having sealing engagement with said relatively rotatable surface; and a plurality of bearing elements projecting from said circular body of sealing material and being circumferentially distributed about said circular body of sealing material and having bearing engagement with said relatively rotatable surface.
The present invention builds on the technology disclosed in U.S. Pat. No. 6,036,192 to provide the xe2x80x9cplurality of bearing elementsxe2x80x9d described therein with a configuration for providing elevated contact pressure zones of skewed configuration to improve lubricant distribution across the dynamic sealing interface.
The present invention is a resilient, generally ring shaped hydrodynamic rotary seal for partitioning a lubricant from an environment, and for hydrodynamically lubricating the dynamic sealing interface between the seal and a shaft. One object of the invention is improved exclusion-edge circularity compared to prior art, particularly when the environment pressure is higher than the lubricant pressure, and also when the lubricant and the environment are substantially equal in pressure. Another object of the invention improved distribution of lubricant across the dynamic sealing interface, compared to the prior art, by use of elevated contact pressure zones which may be skewed with respect to the direction of relative rotation. Another object of the invention is providing support against extrusion when the lubricant pressure is higher than the environment pressure. Another object of the invention is to provide a dynamic sealing surface and a dynamic sealing lip which are robust, providing sufficient width to both to be able to perform for long periods of time in high differential pressure applications and in applications having high concentrations of abrasives in the environment.
Circularity of the environment-side edge of the dynamic sealing interface is critical from an environment exclusion standpoint. In prior art compression-type hydrodynamic seals, such circularity has been difficult to achieve under some operating conditions due to assorted influences of the hydrodynamic wave shape. In the preferred embodiment of the present invention, improved environment-edge circularity is achieved by incorporating a dynamic sealing lip, which is substantially unvarying in width, thereby eliminating uneven deflection in response to pressure. The environment-side of the seal is configured to be supported by an environment side-gland wall when the lubricant pressure is higher than the environment. In applications where the lubricant pressure and environment pressure are substantially equal, the seal can be constrained by a seal groove to resist circumferential-compression-induced buckling and thereby assure environment-edge circularity. This invention, especially when implemented in an axially constrained manner, can provide improved environmental exclusion performance compared to the prior art discussed above.
In the present invention, elevated contact pressure zones caused by pressure manipulation features are used to provide improved lubricant distribution across the dynamic sealing interface. At least a portion of each of the pressure manipulation features is located on the dynamic sealing surface of the seal. The pressure manipulation features are also capable of initiating hydrodynamic lubrication, thereby eliminating the need for conventional hydrodynamic waves, thus allowing a substantially constant dynamic sealing lip width and dynamic sealing surface width.
The elevated contact pressure zones can be produced by various manner of pressure manipulation features, such as projecting rounded or pointed features, such as ledges, and such as recessed local depressions, all of which may elongated, and all of which may be skewed in relation to the direction of relative rotation.
The elevated contact pressure zones cause diversion of the lubricant film toward the environment. Some lubricant leaks past the elevated contact pressure zones, thereby assuring adequate lubrication thereof. The elevated contact pressure zones cause a net leakage of lubricant film into the environment.
This invention has application where relatively rotatable surfaces are sealed with respect to a structure with either the structure or the relatively rotatable surface, or both, being the rotary member. The dynamic sealing surface can be substantially radially oriented when the seal is compressed in the radial direction between substantially cylindrical counter-surfaces, with the dynamic sealing surface and the pressure manipulation features being located on either the inner or the outer periphery of the seal as required by the application.
Alternatively, the dynamic sealing surface can be substantially axially oriented when the seal is compressed in the axial direction between relatively rotating substantially planar counter-surfaces. In such face-sealing arrangements, the pressure manipulation features can be oriented to pump in an outward direction if the lubricant is located inward of the dynamic sealing surface, and can be oriented to pump in an inward direction if the lubricant is located outward of the dynamic sealing surface.
In this invention, hydrodynamic action can be created by means other than large molded-in waves so that (1) the dynamic lip is substantially the same strength and volume about it""s circumference such that all portions of the lip deflect substantially equally under pressure induced from the environment side so that no hydrodynamics are introduced from the environment side as a function of differential pressure acting across the seal from the environment side, and (2) so that the hydrodynamic action is not substantially diminished by circumferential relief of differential thermal expansion.
The invention provides a method for establishing a sealed partition between an environment and a lubricant, comprising: (a) providing a seal having a generally ring-shaped body defining a dynamic sealing surface of generally circular configuration having a plurality of pressure manipulation features projecting therefrom; (b) providing a relatively rotatable surface having a direction of relative rotation relative to said dynamic sealing surface; and (c) compressing said dynamic sealing surface and said plurality of pressure manipulation features against said relatively rotatable surface, wherein: (i) said dynamic sealing surface establishing sealing contact pressure with said relatively rotatable surface, thereby establishing a dynamic sealing interface between said dynamic sealing surface and said relatively rotatable surface; (ii) said plurality of pressure manipulating features establishing at least one elevated contact pressure zone with said relatively rotatable surface being at a contact pressure that is higher than said sealing contact pressure; and (iii) said at least one elevated contact pressure zone being skewed with respect to said direction of relative rotation.
The invention also provides a method for controlling lubricant movement within a dynamic sealing interface between a generally ring-shaped seal and a relatively rotatable surface, wherein the seal establishes a partition between a lubricant and an environment and has a dynamic sealing surface establishing a dynamic sealing interface with the relatively rotatable surface and has a plurality of pressure manipulating features projecting from the dynamic sealing surface, said method comprising: (a) with said plurality of pressure manipulating features, collectively establishing at least one, and preferably a plurality of, elevated contact pressure zones with said relatively rotatable surface and within said dynamic sealing interface; and (b) with said elevated contact pressure zones, causing directionally controlled movement of lubricant within said dynamic sealing interface and toward the environment. It is preferred that the elevated contact pressure zones be skewed with respect to the direction of relative rotation. The above method can be further expanded further include having the seal and the relatively rotatable surface establish at least one direction of relative rotation, said plurality of pressure manipulating features being disposed in skewed relation with respect to said at least one direction of relative rotation, said method comprising: causing interaction of said plurality of pressure manipulating features with said lubricant within said dynamic sealing interface so that said skewed relation thereof develops said directionally controlled movement of lubricant responsive to relative rotation between said generally ring-shaped seal and said relatively rotatable surface.